Systems of this character, known in the trade as mammoth pumps, include two intercommunicating conduits, namely a first and a second tube, which are used to create a pressure differential with the aid of bubbles of air or some other gas injected into the second or riser tube near the foot thereof in order to reduce the specific weight of its contents. Thus, a slurry or other liquid containing solid particles can be introduced from above into the shorter first tube to descend therein while acting as a hydrostatic column letting the same slurry, lightened by the introduced gas bubbles, ascend in the longer second tube to the top of the latter for transportation of the particles to the higher level. This type of system is generally utilized for the conveyance of finely comminuted material such as activated charcoal from an adsorber bed to a regenerator from which that material is subsequently returned to the bed. The utilization of a slurry has the advantage of minimizing the abrasion undergone by the particles during transportation.
In large installations, such as clearing plants for the purification of waste water, the difference in elevation between the pick-up level and the discharge level of the conveyed particles often has to be greater than that available with the conventional pumping system described. The extent to which the second or riser tube can project above the top of the first or descending tube, forming the hydrostatic column, is determined by the proportion of the injected gas which must be kept low enough to preserve the abrasion-minimizing effect of the slurry; the height of the second tube, in fact, can never by more than twice that of the first one. To satisfy the requirement for a large level difference, therefore, a considerable overdimensioning of the conduits--also taking into account the increased flow resistance--would be required.